Bushing insulator for core type induction furnace



6- F. KOLLE Dec. 7, 1965 BUSHING INSULATOR FOR CORE TYPE INDUCTION FURNACE Filed July 24, 1962 F/GZ INVENTOR. GEO/PG! FEEDER/C K01. LE

Wan/MSW United States Patent 3,222,446 BUSHING INSULATOR FOR CORE TYPE INDUCTION FURNACE George F. Kolle, Yardley, Pa., assignor to Inductotherm Linemelt Corp., Rancocas, N.J., a corporation of New Jersey Filed July 24, 1962, Ser. No. 212,032 7 Claims. (Cl. 13-29) In general, this invention relates to a new and improved insulated bushing for a core type induction furnace and, more particularly, to an insulated bushing for the air duct of a core type induction furnace which is manufactured from two materials.

Induction furnaces of the core type have been in successful operation for more than forty years for the melting of many metals and their alloys. One of the problems inherent in this type of furnace has been the inability to melt lead-rich copper alloys containing more than of lead. When high-lead alloys are melted, the lead rapidly penetrates the lining short-circuiting the primary and causing an overload. Previous studie have involved the problems inherent in melting lead-rich and particularly lead-rich copper and lead-rich copper tin alloys in core type induction furnaces.

These alloys, and particularly the tin containing alloys, form in the furnace and particularly within the refractory lining adjacent to the molten metal, low melting segregations of easy fluidity and of a composition which is entirely different from that of the molten charge. The lead and tin contents of the segregations are very high and much higher than that of the metal charged in the furnace. The melting point is sometimes as low as about 600 F. and less. The thinly fluid lead and tinrich segregations accumulate in the secondary slot and penetrate from and through the smallest cavities or pores of the secondary block towards the cooled duct in which the primary is located. As soon as even a slight penetration has occurred, a quick deterioration and breakthrough of the refractory block follows very shortly with the result that the coils of the primary circuit are often attacked and destroyed. The exudations of low melting temperature penetrating through the refractory walls of the secondary block tend in many cases also to form a second ring of metal around the primary. As soon as a new secondary is formed in this manner, more electrical current is absorbed and the process of deterioration is accelerated. In the past, this problem has been solved by the combination of an increase in the wall thickness or that portion of the refractory block which extends between the secondary slot and the cooling duct, and an increase of the surface coefficient of heat transmission at the cold face of the secondary block.

One of the methods of increasing the surface coefiicient of heat transmission at the cold face of the secondary block has been the introduction of a split metallic liner on the cold face of the metallic block. This metallic liner has a high thermal conductivity. It is split and insulated in order for it not to become a secondary circuit about the primary core and coil. The liner functions are threefold: (1) it is used as a ramming mold in order to form the necessary path for threading the primary core and coil, (2) it is used to disperse the heat of the lining evenly throughout the air stream and thus avoid hot spots on the refractory and coil, and (3) minute leakage of metal to the air duct liner, or bushing, is frozen against the essentially cold bushing and, thus, service life of the furnace can be enhanced.

Since these bushings must be electrically split, or in- 3,222,446 Patented Dec. 7, 1965 sulated, it was traditional to use an insulator at the split in the bushing. These insulators, while being important to the electrical operation of the unit, subtract from the purpose of the bushing, i.e. the even distribution of heat in the lining. In some cases, where cracks through the lining allow metal to bridge the insulation, the bushing short circuits, conducts electricity, and leads to very prompt furnace failure. The fact that these electrical insulators have poor thermal conductivity promotes the leakage of metal towards them as they are effectively running hotter than the highly conductive bushing. This insulator in the prior art was usually made of an asbestos cement or any other suitable insulator.

The problem with the bushing discussed above has been recognized by persons working in this art. To overcome this problem, the bushings with their electrical insulators have been wrapped with mica and/or fiber glass and varnish in order to 1) equalize the heat conductivity of the bushing, and (2) give the bushing more mechanical strength and electrical insulation.

However, all of the insulating materials discussed above detract from the stated purpose of evenly distributing and effectively cooling the refractory into the air stream. It may also be stated, if a bushing is used uninsulated, it is rather difiicult to obtain a good bond between the refractory and the bushing. This results in a poor thermal conductivity or sliding of the bushing into and out of position depending on the attitude of the furnace. In such cases, it is obvious that the bushing is serving little or no good purpose and, in fact, can be quite detrimental. Insulated bushings suffer not only from the fact that they hamper the essential functions of the bushing, but also insulating materials can easily be torn or at best make a very poor bond with refractory when the furnace is being lined.

To make the bushings of a non-metallic material offering high resistance to temperature and molten metal attack, having high thermal conductivity, and high electrical resistance is not practical. Such non-metallic structures can only be manufactured of materials which are highly fragile and do not offer mechanical strength necessary for their task.

It is therefore the primary object of this invention to construct a core type induction furnace where the penetration of the secondary block by low melting segregations and/ or exudations formed from the molten charge is prevented.

Another object is to provide a new and better insulated bushing having a layer of an electrically nonconductive material bonded thereto which will have electrically insulating properties and high thermal conductivity.

Another object of this invention is to provide a new and improved liner for a core type induction furnace which is mechanically stronger, electrically insulating, and thermally better conductive, and provides a better surface to stop or freeze molten leakage through the refractory.

Another object of this invention is to provide a liner for a core type induction furnace which is stronger than the prior art liners as it has a metallurgically bonded coating which has a bond strength greater than any varnish or plastic type insulations at operating temperatures.

Another object of this invention is to provide a simple and better liner for a core type furnace which has a metallurgically bonded non-metallic insulator to which the refractory material can better adhere than the prior art liners.

Other objects will appear hereinafter.

For the purpose of illustrating the invention there is shown in the drawings a form which is presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown.

FIGURE 1 is a vertical sectional elevation of a furnace incorporating the teachings of the present invention.

FIGURE 2 is a vertical sectional elevation of the furnace of FIGURE 1 taken along lines 22.

FIGURE 3 is a perspective view of the liner used between the secondary block and the air conduit incorporating the features of the present invention.

In the drawings, there is shown a housing containing a hearth 12 adapted to hold the molten metal and provided in the usual manner with a refractory lining 14. Suitable insulating material may be inserted between the lining 14 and the housing 10. The metal forms a charge which is heated by means of currents produced in a secondary slot 24 formed as a loop in a secondary block 16. The loop 24 is threaded by a primary circuit composed of iron core 26 and surrounded by primary coil 20. The primary coil may be made of a flat wound copper wire, rectangular in cross section insulated and wound in layers. A flat wound multilayer coil develops less heat than an edge wound single layer coil occupying the same place and, therefore, helps in reducing the temperature prevailing at the cold face of the refractory block.

The hot face of the refractory block 16 is designated by the numeral 18. The inner cylindrical face of the block 16 is the cold face and is rammed against a liner 22. A cooling duct 28 is provided between the primary coil 20, core 26, and the liner 22. Air is drawn through the cooling duct 28 by a motor driven blower 34. By varying the rotational speed of the blower, the velocity of the cooling air can be increased and decreased.

The air cooling ducts of induction furnaces of the type shown in FIGURE 1 are not simple in shape like ordinary tubular ducts. The passage 28 of air is a small cavity located between the outside surface of the refractory liner 22 and the transformer. The transformer consists of two parts, namely the core 26 and the coil 20, and the air will flow through the passages formed between them also.

In the case of liquid cooled furnaces, the liner 22 would also be necessary.

FIGURE 3 shows further details of the bushing or liner 22 in perspective. It consists of an inner split cylin drical former made of metal sheet rolled to size. The split cylinder 30 has a longitudinal slot running the total axial length thereof to split the cylinder 30 and prevent it from acting as a coil. The metallic split cylinder acts as a good heat conducting material having a very large surface exposed to the cooling air stream. Thus, it has good heat transfer characteristics.

A coating 28 of alumina is metallurgically bonded to the split metallic cylinder 30. The alumina coating 28 also fills the longitudinal slot in the split cylinder 30 to form a bridge 32.

The alumina coating 28 has a substantial thickness of to of an inch. It is to be understood that other refractory materials offering high resistance to temperature and molten metal attack and having high thermal conductivity, high electrical resistance, providing a surface with high resistance to mechanical abrasion, and an excellent bonding surface for the refractory may be utilized.

Metallurgical bonding of the refractory material to the metallic layer implies (1) bonding by direct physical or chemical fusion, (2) flame deposition, (3) electrodeposition and fusion, (4) flame deposition and fusion. An example would be flame deposited alumina on copper.

By utilizing a refractory material rather than asbestos or the like as an insulator, the thermal conductivity of the split cylinder 30 can more closely be approached. Thus, electrical insulator coating 28 having the same thermal conductivity as the metal will not promote the leakage of metal towards the insulator as it will operate at approximately the same temperature as the highly conductive split metal cylinder 30. As the non-metallic material coating 28 offers a high resistance to temperature and molten metal attack, it will provide a better surface to stop or freeze molten leakage through the base refractory.

When the molten metal has been heated in the annular secondary slot 24, the entire induction furnace is then rotated on trunions 36 and 38 and the metal poured from spout 40.

The present invention may be embodied in other specific forms without departing from the spir-t or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification as indicating the scope of the invention.

I claim:

1. An electric induction furnace for molten metal comprising a metal holding hearth, an induction heating unit including a secondary block of refractory material having a loop channel therein for receiving molten metal, said loop channel being in communication with said hearth, said secondary block having an opening therethrough centrally of said loop channel and defining a cold face on said secondary block, a primary core and coil for said induction heating unit located within said opening, a heat conductive, non-magnetic, metal, split, hollow, elongated former of uniform cross sections having high electrical conductivity, said former having an axial slot, said metallic former having a refractory electrical insulator material of high thermal-conductivity within said slot along the entire length of said slot to prevent the formation of a complete electrical circuit in said metallic former and permit heat transfer, said fromer and said insulator being held against said cold face, and cooling means for cooling said former and insulator located between said former and said primary core and coil.

2. An electric induction furnace for molten metal comprising a metal holding hearth, an induction heating unit including a secondary block of refractory material having a loop channel therein for receiving molten metal, said loop channel being in communication with said hearth, said secondary block having an opening therethrough centrally of said loop channel and defining a cold face on said secondary block, a primary core and coil for said induction heating unit located within said opening, an annular liner, said liner having its entire outer peripheral surface of a thermally conductive electrically insulating refractory material, said liner having an inner surface of a thermally conductive non-magnetic metallic split cylinder, means for electrically insulating the length of the split in said cylinder to prevent the formation of a complete electrical circuit in said metal cylinder, said liner being held in place against said cold face, and a cooling means for cooling said liner and insulator operative on the inner surface of said liner.

3. The electric induction furnace of claim 2 wherein said insulating means is a refractory insulator of high thermal and low electrical conductivity, said refractory material and said insulator being metallurgically bonded to said metal layer.

4. The electric induction furnace of claim 3 wherein said refractory material and insulator are alumina.

5. A liner for a core-type electric induction furnace comprising a split, thermally conductive, non-magnetic, hollow, elongated, metal former of uniform cross section, said former having a split along the axial length thereof, and a thermally conductive, electrically insulating refractory material on the entire outer peripheral surface of said former and in said split, said refractory material being metallurgically bonded to said metal former, said refractory material having a thermal conductivity approaching the thermal conductivity of the former.

6. A liner in accordance with claim 5 wherein said refractory material is alumina andsaid former is copper.

References Cited by the Examiner UNITED STATES PATENTS Osterheld 338-245 Seaman 338-245 Tama et al 13-29 Montgomery et a1. 117-46 Fenn 219-270 Cooke 13-29 10 RICHARD M. WOOD, Primary Examiner. 

5. A LINER FOR A CORE-TYPE ELECTRIC INDUCTION FURNACE COMPRISING A SPLIT, THERMALLY CONDUCTIVE, NON-MAGNETIC, HOLLOW, ELONGATED, METAL FORMER OF UNIFORM CROSS SECTION, SAID FORMER HAVING A SPLIT ALONG THE AXIAL LENGTH THEREOF, AND A THERMALLY CONDUCTIVE, ELECTRICALLY INSULATING REFRACTORY MATERIAL ON THE ENTIRE OUTER PERIPHERAL SURFACE OF SAID FORMER AND IN SAID SPLIT, SAID REFRACTORY MATERIAL BEING METALLURGICALLY BONDED TO SAID METAL FORMER, SAID REFRACTORY MATERIAL HAVING A THERMAL CONDUCTIVITY APPROCHING THE THERMAL CONDUCTIVITY OF THE FORMER. 